1. Field of the Invention
One embodiment of the invention relates to a defacement degree determination apparatus and defacement degree determination method, which determine, for example, a defacement degree of the surface of a printed material such as securities based on a two-dimensional image of the printed material.
2. Description of the Related Art
In recent years, a demand for determining a defacement degree of a printed material based on a two-dimensional image of the printed material obtained by image input means such as a camera or scanner is increasing, and many methods of defacement degree determination by executing arithmetic processing of sampling values from images have been proposed. For example, a method of determining a defacement degree based on the average or variance of density values sampled from a printed part of a printed material, a method of making similar determination conversely from a non-printed part of a printed material, or a method of combining these methods is available.
Jpn. Pat. Appln. KOKAI Publication No. 2000-62299 discloses a technique associated with such defacement degree determination of a printed material. With the disclosed technique, a printed material is aligned based on an image of the printed material. Defacement degree determination values are calculated by executing arithmetic (integration) processing, based on at least two or more pieces of region information which are stored in advance, from values obtained by normalizing the differences between the obtained density average values and a predetermined standard value by a standard deviation, the areas of the regions, and visual characteristic coefficients of the regions (coefficients corresponding to different defacement degrees that one feels for a light printed region and dark printed region). Then, a final defacement degree is calculated by aggregating them, and the defacement degree is determined using a threshold.
Note that the region information is obtained by applying threshold processing for each print type (light or dark) based on the average value of a plurality of image data, but it is based only on the density. Alignment and rotation correction of an image are based on a known method. Furthermore, as for aggregation of the defacement degree determination values, only a method by means of simple arithmetic operations (additions) is disclosed.
In the aforementioned related art, a practical problem is the alignment precision. Conventionally, the edge (a position where the density changes largely) of a printed material is detected using a technique like a differential filter, and alignment is done with reference to that edge. However, a sufficiently high precision cannot often be obtained due to deficiency or deformation of the edge part.
A material printed using a plurality of printing plates includes a plurality of printed components corresponding to these printing plates. For this reason, due to the influence of relative positional variations of these printing plates, the plurality of printed components may also suffer relative positional variations. The aforementioned related art does not consider such positional variations.
Furthermore, the resolution of image input means has limitations of principle. Even for an identical printed material, the positions of input images vary due to slight positional deviations of a pixel pitch or less when an image is input a plurality of times. However, since the related art cannot attain alignment to a precision exceeding the pixel pitch, it cannot follow up such variations.
For some reasons described above, sampling values from images unwantedly become unstable in the related art.
The way defacement of a printed material advances is closely related to the nature of a printing plate. For example, in case of an intaglio plate (intaglio printing plate), since the density of printing is expressed by the thickness of ink (a printed part is dark if the ink is thick; it is light if the ink is thin), the ink of a dark part is shaved as the defacement advances, and the contrast becomes low in general.
On the other hand, in case of a planographic plate (planographic printing plate), the ink is rarely shaved as the defacement advances, and color tone changes due to adhesion of dirt or aging of the printed surface. In this manner, in order to detect a change of the printed surface that appears as the defacement advances, a sampling method suited to each printing plate should be selected. However, in the related art, although regions for respective print types are defined, sampling values from images are only densities, and the defacement of a printed material cannot be precisely detected.
Furthermore, rotation correction of an image as a previous step of alignment is based on a known method as in alignment, and rotation correction with a sufficiently high precision cannot often be done for the same reasons as those described for the alignment.